A detailed study of the interaction between high-speed gas flows and surface dielectric barrier discharges (DBD) is presented. In the present paper, it is demonstrated that a DBD can be sustained in transonic airflows, up to isentropic Mach numbers of 1.1. The plasma is characterized electrically, as well as optically with a CCD camera and a photomultiplier tube. Different airflow velocities, plasma excitation frequencies and voltages are investigated. The airflow has a significant influence on the plasma characteristics: the glow component is reduced, the discharge becomes more filamentary and most importantly, the light emission duration from individual microdischarges is reduced by more than a factor of ten at high flow velocities. Large edge effects play a key role in the interaction between the flow and the plasma. These results offer new perspectives for the use of dielectric barrier discharges in transonic and supersonic gas flows and their applications to airflow control and to plasma-assisted combustion.